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Accused, Yes, but Probably Not a Killer

AFTER 20 years of cultivating my garden in sunny inland Palo Alto, last year I moved 40 miles to San Francisco, where the cold, foggy summers are less hospitable to heat-loving tomatoes. Here at the end of July, my tomato plants are bearing just a few green fruits. But they have lots of healthy foliage. The leaves may not taste like ripe tomatoes, but they do have tomato flavor. Which made me think, why not cook more with tomato leaves?

Because they’re poisonous! That’s what most people have said when I mention the idea. But it turns out that there’s little good evidence for that common belief. So I’ve branched out from Paul Bertolli’s leaf-flavored pasta sauce, an old favorite. I think tomato leaves are worth adding to our roster of kitchen herbs.

There are reasons to be wary of the tomato plant. It belongs to the disreputable nightshade family, whose members accumulate toxic alkaloids. For centuries after the Spanish first took the tomato from Mexico to Europe, fruit and plant alike were considered dangerous. Nowadays the fruit is summer’s star, but the rest of the plant is still suspect.

Unfortunately there’s no authoritative roster of poisonous plants to consult for definitive advice about edibility. The Food and Drug Administration maintains an online poisonous plant database, but with the disclaimer that it has “no official status” because the information it includes is unconfirmed and constantly changing. So out-of-date and erroneous materials persist. Toxicity is an inexact quality in any case, because it depends on dosage and other variables.

Many handbooks of poisonous plants cite the tomato plant for killing livestock and sickening people. According to the California Poison Control System’s “Poisoning and Drug Overdose,” edited by Kent R. Olson (McGraw-Hill, 2006), the tomato toxin is solanine, one of two alkaloids that make greened potatoes toxic. High doses of solanine kill animals and cause nausea, hallucinations and death in humans.

Sounds pretty damning. But there’s scant evidence for tomato toxicity in the medical and veterinary literature. I found just one medical case, an undocumented reference to children having been made sick by tomato-leaf tea, in a 1974 book on poisonous plants. In contrast to the few anecdotal accounts of livestock poisoning, a controlled study in Israel in 1996 showed no ill effects when cattle ate tomato vines for 42 days.

And it’s a chemical gaffe to attribute tomato toxicity to solanine. Dr. Mendel Friedman of the federal Department of Agriculture, who has studied potato and tomato alkaloids for two decades, wrote in an e-mail message that commercial tomatoes contain tomatine. Solanine, he added, is a potato alkaloid.

There are significant quantities of tomatine in green tomato fruits, which people have long eaten fried and pickled. And tomatine appears to be a relatively benign alkaloid.

In 2000, Dr. Friedman and colleagues reported that when lab animals ingest tomatine, essentially all of it passes through the animal unabsorbed. The alkaloid apparently binds to cholesterol in the digestive system, and the combination is excreted — ridding the body of both alkaloid and cholesterol. The researchers found that both tomatine-rich green tomatoes and purified tomatine lowered the levels of undesirable LDL cholesterol in animals.

Dr. Friedman has also found that an extract of green tomato lowers the incidence of cancer in animals, and last month he reported that both this extract and purified tomatine inhibit the growth of various human cancer cells. Other studies have found that purified tomatine seems to stimulate the immune system in desirable ways.

According to “Toxic Plants of North America” (Iowa State University Press, 2001), by George E. Burrows and Ronald J. Tyrl, a toxic dose of tomatine for an adult human would appear to require at least a pound of tomato leaves. These authors conclude that “the hazard in most situations is low.”

Photo

GO AHEAD AND PICKLE An alkaloid in green tomatoes seems to do people no harm.Credit
Francesco Tonelli for The New York Times

If tomato leaves don’t deserve their toxic reputation, then what can I do with them? It’s remarkable how cooks have ignored them, even in the region where the tomato was first eaten. I checked with Diana Kennedy, the maestra of Mexican food culture, and Maricel Presilla, a chef and writer who has traveled extensively for her eagerly anticipated treatise on Latin American cooking. Neither had come across tomato leaves in the kitchen.

Many European writers openly disapproved of the peculiarly strong smell of the raw leaves. In his 1731 “Gardener’s Dictionary,” Philip Miller wrote that “the plants emit so strong an effluvium as renders them unfit to stand near an habitation, or any other place that is much frequented.”

What smells rank in the air can taste good in a sauce. I learned this years ago from Paul Bertolli, a chef and salumi maker in Berkeley, Calif., who set aside the received wisdom and called for tomato greens in his book “Cooking by Hand” (Clarkson Potter, 2003).

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“I started using the tomato leaves to punch up the flavor of my quick tomato sauces at Chez Panisse around 1987,” Mr. Bertolli recently explained in an e-mail message. “I found them very effective in offering up that just-picked, viney, tomato taste.”

“One year a family of deer jumped my fence and munched on my tomato crop,” he added. “The leaves didn’t seem to deter them, in fact, they came back for more the second day. I tried a little in a sauce and I also came back the second day with no noticeable ill effects. From then on, I have used them steadily.”

Apart from Mr. Bertolli’s excellent leaf-enhanced sauce, I’ve found only a handful of obscure uses for tomato leaves, all of them from Asia. Exploring the East Indies in the 17th century, the Dutch botanist G. E. Rumpf noted that the people of Ambon Island, now part of Indonesia, ate the tender leaves raw with fish and with fermented shellfish, a precursor of Indonesian belacan and relative of Asian fish sauces. Later, the botanist J. K. Hasskarl found the young leaves eaten along with rice. But Sri Owen, an Indonesian food writer, told me by e-mail that she has never heard of any such dishes.

More recently, on an episode of the original Japanese “Iron Chef” in 2000, the chef Hiroyuki Sakai served raw fish in a sauce that included dried tomato leaves. And there’s now a Japanese patent pending for a process that dries tomato plants and grinds them into an antioxidant-rich food powder.

With these examples in mind, I tried combining finely shredded tomato leaves and just a hint of fish sauce, and found that they made a savory garnish for both rice and pan-cooked halibut. Then I gently fried whole leaves for a few seconds on each side, and they came out crisp and beautifully translucent, delicious sprinkled with a few grains of salt. Dried, they taste like tea. Blanched and puréed, a few spoonfuls of tomato leaf added deep green flavor and color to a pesto. No side effects noted.

Pondering safety while making pesto prompted me to do a background check on basil. It doesn’t contain any alkaloids, but two of the chemical components in its aroma have been found to cause DNA damage and cancer in animals. These substances, estragole and methyleugenol, are also found in other herbs and are added to manufactured foods. A European food safety agency has proposed regulating their use.

There’s no evidence that eating pesto is hazardous. Researchers at Wageningen University in the Netherlands and the Nestlé Research Center in Lausanne, Switzerland, found that an extract of the whole basil leaf can block the DNA damage caused by estragole.

But the ongoing stories of tomato leaves and basil show how little we really know about what we eat. Plant foods contain many thousands of different chemicals, and each one can have a number of different effects on the body, some benign, others not.

We’ve come to eat what we eat based on long but limited experience, and scant understanding. As we learn more, maybe tomato leaves will become a mainstream ingredient. Or maybe not.

Meanwhile, as I squint from the window to make out any hint of orange on the plants huddled in the swirling fog, it’s good to know that ripeness isn’t all.

A version of this article appears in print on , on Page D1 of the New York edition with the headline: Accused, Yes, but Probably Not a Killer. Order Reprints|Today's Paper|Subscribe